Multiple-site RF transmission systems broadcast signals from more than one base station. This allows radio communications to cover a larger area than is possible with a single base station.
The present invention relates to a network of several single site trunked radio systems. An example of a single site transceiver system is disclosed in commonly-assigned U.S. Pat. No. 4,905,302, entitled "Trunked Radio Repeater System" and U.S. Pat. No. 4,903,321 entitled "Radio Trunking Fault Detection System" which are incorporated by reference. Digital trunked radio transceivers capable of handling communications between numerous mobile units and dispatcher consoles in a single area are known.
It is generally impractical for a single VHF/UHF RF repeater transmitting site to effectively serve a large geographical area. The broadcast area of a single site is limited by several factors. The effective radiated power of the antenna is subject to legal and practical limits. In addition, natural and man-made topographical features, such as mountains and buildings, that block RF signal from certain locations.
Multiple transmitting sites are necessary to provide RF communications to all locations within a given locality. Multiple transmitters may be needed to cover a rural community covering many square miles or a city having tall buildings that block RF transmissions. FIG. 1 is a schematic diagram of a simplified multiple-site system having three radio repeater (transmitting) central sites S1, S2, and S3 providing communications to geographic areas A1, A2, and A3, respectively. Mobile or portable transceivers within area A1 receive signals transmitted by site S1, transceivers within area A2 receive signals transmitted by site S2, and transceivers within area A3 receive signals transmitted by site S3. Each site has a site controller that acts as a central point for communications in the site. To enable communications from one area to another, a switch networks the radio systems together to establish audio slots connecting one site controller to another. Thus, a caller in one area can communicate with someone in another area.
The present invention is directed to a multisite RF trunked repeater system that allows a caller in one site area (e.g. A1) to communicate with a caller in another area (e.g. A2). In a multisite network, each site assigns a specific channel to a call independently of the channel assignments made by other sites. Thus, a single call may be broadcast from several site transmitters each operating on a different frequency. A central multisite switch routes audio and command signals from one site to another, and to and from dispatcher consoles.
In multisite, the site controller (S1) receives a call from a mobile radio in A1 requesting a channel to communicate with a specific caller. A caller requests a channel simply by pressing the push-to-talk (PTT) button on his microphone. This informs the site controller that a channel is requested. The PTT signal is transmitted to the unit on a control channel that is continuously monitored by the site controller. The site controller assigns a channel to the call and instructs the caller's radio unit to switch from the control channel to the channel assigned to the call. This assigned channel is applicable only within the area covered by the site.
In addition, the site controller sends the channel assignment to the multisite network switch. The switch assigns one of its internal audio slots to the call. The switch also sends a channel request to all other site controllers or to only those site controllers having a designated caller within its area. Upon receiving a channel request, these secondary site controllers assign a channel to the call. Again, each secondary channel is operative only in the area covered by its secondary site controller. Each secondary site controller also sends a channel assignment back to the multisite switch. The switch connects the site controller line carrying the assigned channel to the assigned audio slot. The caller can then communicate with a unit or group in an other area via the multisite switch. The call is initially transmitted to the primary (host) site controller, routed through the assigned audio slot in the switch and retransmitted by the secondary sites on various assigned channels in those other areas.
When the caller ends the call, the primary site controller deactivates the assigned channel for that site and notifies the network switch that the call is terminated. There may be a brief "hang time" after the end of the call during which the channel remains assigned. During this hang time, the call can be rekeyed without going through the channel assignment procedure.
When the call is dropped, the network switch sends an end of call command (slot idle) to the secondary site controllers. A call is terminated in a similar format and operation as the slot assignment. Instead of establishing an audio slot, the end of call command causes the assigned slots and channels to be released.
In addition to providing communications between mobile radio units in different areas, the multisite network switch provides communications between dispatchers in different areas and between dispatchers and mobile radio units in different areas. The dispatcher consoles are connected to the network switch in the same manner as are the site controllers. A dispatcher console can issue a channel call request through the network switch to a site controller in another area to call a mobile unit or to another dispatcher console to call a dispatcher at another console.
In addition to all of the features that the mobile units have, each dispatcher console has the ability to participate in any call in its area or to its assigned groups. Thus, when a call comes through the network switch from another area to a mobile radio, the network switch informs the dispatcher console of the call in addition to notifying the site controller. The dispatcher can listen in or participate in the call to the mobile radio.
The network switch is also capable of handling calls to groups of mobile units and/or dispatcher consoles. The wide area switch manages group calls and monitors the network to ensure that the site controllers for all of the callers in the group assign a channel to the group call. If a channel is not assigned, the wide area switch advises the caller that the wide area call cannot be formed as requested. The caller then has the option of re-keying the call so as to reach those areas having assigned channels.
The present invention relates to a multisite switch having a distributed architecture. The logical functions of the switch are shared by various microprocessor operated nodes distributed throughout the switch. Each node includes a controller card and audio cards. The nodes share the computational workload of the switch. Within the switch, the nodes are coupled to each other by message and audio buses.
The nodes interface the switch with the other radio system components outside of the switch. Each node is connected to a site controller, dispatcher console, the system manager or other component of the overall radio system. The nodes coupled to site controllers are referred to as Master II Interface Modules (MIMs) and the nodes coupled to dispatcher consoles are referred to as Console Interface Modules (CIMs).
Distributed network multisite systems have a much faster data transfer rate than comparable central architecture multisite systems. Central computers process information serially. All communications passing through the switch must be serially processed by the central computer. The central computer slows communications because of its serial operation. Distributed network systems achieve parallel processing by sharing the computational tasks between several processors. Distributed networks are generally significantly faster than central computers.
In the multisite switch, the overall protocol used on the message bus is the carrier-sense multi-access protocol with collision detection (CSMA/CD). This is a peer-to-peer type protocol in which all of the nodes interact as equal logical units in the locate area network (LAN). There is no arbiter unit in the switch to resolve conflicts between nodes. Such an arbiter was undesirable because its failure would disable the entire switch. An underlying design principal of the switch was to eliminate any single unit that could disable the entire switch. Accordingly, there is no arbiter unit in the multisite switch and the switch nodes together and by themselves control the operation of the switch and LAN.
Each node in the switch must have a unique node address. These addresses identify the nodes and address tables in which the node addresses are stored. Messages generally include the address of the node that sent the message so the other nodes can determine the origin of the message. In addition, some messages are intended for certain nodes and these messages include the address of the intended recipient node to ensure that the message is received by the designated node.
In the multisite switch of the present invention, the assignment of node addresses is accomplished through a novel method by which each node assigns itself a randomly-selected address and then broadcasts its address to other nodes to determine whether its address was also selected by another node. If two or more nodes have the same address, then these nodes reassign themselves new addresses. Accordingly, whenever the switch is booted-up or reset, the nodes follow a procedure by which they each allocate themselves a unique address. Similarly, when a new node is added to the switch, it selects its own address and broadcasts this address to other nodes to confirm that its address is unique.
One advantage of this method of assigning node addresses is that it does not require an arbiter unit or other central unit to assign addresses. Similarly, there is no requirement for a human operator to assign node addresses or set up each node individually in some special manner, such as by setting DIP switches. The distributed architecture of the switch is preserved by the present method of dynamically allocating node addresses. There is no multisite switch component that is required to implement the addressing algorithm. The nodes assign themselves unique addresses regardless if any node or other switch component is inoperative. Accordingly, there is no one component that can prevent the dynamic addressing method of the present invention.